Usually non-critical string theory is considered in frames of the approach proposed by Polyakov. The other approach has been developed in. It represents a universal method to maintain explicit Lorentz invariance in any quantum relativistic theory. On an example of Nambu-Goto string theory in 4-dimensional Minkowski space-time the idea can be demonstrated as follows: * explicit Lorentz-invariance at d=4 * reparametrization degrees of freedom fixed by the gauge * Regge-like spin-mass spectrum